One technology that is used for display systems is based on a digital micromirror device or DMD. Such systems are commercially available from Texas Instruments, Inc. under the trademark DLP™ (Digital Light Processing). Referring to FIG. 1, an example of a DMD system 10 is illustrated, wherein the light from a light source 11 is applied through a first condenser lens 13 and through a color wheel 15, which will typically rotate at least once per frame of the image to be displayed. The light passing through the color wheel 15 passes through a second condenser lens 17 onto a DMD chip 19. The DMD chip includes an array (on the order of one million) of tiny mirror elements, or micro-mirrors, where each mirror element is hinged by a torsion hinge and support post above a memory cell of a CMOS static RAM as shown in FIG. 2.
FIG. 2 shows a portion of a typical DMD array 19 having mirror elements 21 suspended over a substrate 23. Electrostatic attraction between the mirror 21 and an address electrode 25 causes the mirror to twist or pivot, in either of two directions, about an axis formed by a pair of torsion beam hinges 27a and 27b. Typically, the mirror rotates about these hinges until the rotation is mechanically stopped. The movable micro-mirror tilts into the on or off states by electrostatic forces depending on the data written to the cell. The tilt of the mirror is on the order of plus 10 degrees (on) or minus 10 degrees (off) to modulate the light that is incident on the surface. For additional details, see U.S. Pat. No. 5,061,049 entitled “Spatial Light Modulator” and U.S. Pat. No. 5,280,277 entitled “Field Updated Deformable mirror Device,” both by Larry J. Hornbeck.
Referring again to FIG. 1, the light reflected from any of the mirrors may pass through a projection lens 29 and create images on the screen 31. The DMD's are controlled by electronic circuitry fabricated on the silicon substrate 23 under the DMD array. The circuitry includes an array of memory cells, typically one memory cell for each DMD element, connected to the address electrodes 25. The output of a memory cell is connected to one of the two address electrodes and the inverted output of a memory cell is connected to the other address electrode.
Data is provided by a timing and control circuit 33 determined from signal processing circuitry and an image source indicated at 35. Once data is written to each memory cell in the array, a voltage is applied to the DMD mirrors 21 creating a large enough voltage differential between the mirrors 21 and the address electrodes 25 to cause the mirror to rotate or tilt in the direction of the greatest voltage potential. Since the electrostatic attraction grows stronger as the mirror is rotated near an address electrode, the memory cell contents may be changed without altering the position of the mirrors once the mirrors are fully rotated. Thus, the memory cells may be loaded with new data while the array is displaying previous data.
The intensity of each color displayed on the screen 31 is determined by the amount of time the mirror 21 corresponding a particular pixel directs light toward screen 31. For example, each pixel may have 256 intensity levels for each color (e.g., red, green or blue). If the color level selected for a particular pixel at a particular time is 128, then the corresponding mirror would direct light toward that area of screen 31 for ½ (e.g., 128/256) of the frame time.